The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of a rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos ⊖, where ⊖ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
In efforts to meet the ever increasing demands for improved data rate and data capacity, researchers have recently focused on the use of perpendicular recording. In a conventional, longitudinal recording system, data is recorded as magnetic transitions oriented longitudinally on a magnetic disk. In such a longitudinal system, a write element has a write gap disposed between two write poles, and a magnetic fringing field extending from one write pole to the other writes the longitudinal bits of data.
In a perpendicular recording system, however, data is recorded as magnetic transitions oriented perpendicular to the surface of the disk. A perpendicular write element includes a write pole having a very small cross section and a return pole having a much larger cross section. The write pole emits a strong, highly concentrated magnetic field directly toward the disk in a direction perpendicular to the surface of the disk. The disk has a magnetically soft (low coercivity) under layer and a thinner, magnetically hard (high coercivity) upper layer. Since the field emitted from the write pole is very strong and highly concentrated it is sufficient to overcome the high coercivity of the upper layer and thereby magnetize the upper layer. The resulting magnetic flux in the disk travels through the soft underlayer of the disk to the return pole where it passes through the high coercivity upper layer in a much more spread out pattern. Because the magnetic flux at the return pole is sufficiently spread out, it does not overcome the coercivity of the upper layer of the magnetic disk and, therefore, does not erase the bit recorded by the write head.
The write pole of such a perpendicular write head is preferably constructed very small in order to produce a small magnetic bit of data. The use of such small write poles, however, results in magnetic remanence. In a much larger magnetic pole of a traditional longitudinal write head the poles are sufficiently large to allow the formation of magnetic domains. This allows the poles to demagnetize once the field from the coil is removed. However, a perpendicular write pole is so small that such domains cannot form. This means that the write pole cannot demagnetize after the magnetic field from the coil has been removed. A write pole that remains magnetized will inadvertently erase bits of data that were not meant to be erased.
One method for reducing such remanence is to form the write pole as a plurality of laminated layers of magnetic material separated by layers of non magnetic material such as Ru or Cr. This allows the magnetic layers within the write head to form antiparallel domains and thereby demagnetize after the magnetic field from the coil has been removed. Such a write head is described in US Patent Application Publication US2004/0075927 entitled FLUX CLOSED SINGLE POLE PERPENDICULAR HEAD FOR ULTRA NARROW TRACK, which is incorporated herein by reference. Such laminated write poles have been referred to as antiferromatically (AFC) coupled write poles.
One problem that results from the use of such a laminated AFC write pole is that they have a high saturation field. As those skilled in the art will appreciate, the write pole should be magnetically saturated during writing, so a high saturation field (ie. high reluctance and low permeability) results in decreased writing efficiency. The saturation field is directly proportional to the coupling strength between the ferromagnetic layers across the non-magnetic spacer layer. The materials that have been used include CoFe magnetic layers separated by Ru spacer layers. CoFe has been used to provide the necessary high magnetic moment, however as mentioned above, such systems suffer from high saturation field.
Therefore, there remains a need for a write pole for use in a perpendicular recording system that can provide high magnetic moment, and avoid remanence while also having a low saturation field (ie. low reluctance, and high permeability).